Aluminum coated ferrous article



United States Patent 2,782,493 ALUMINUM COATED FERROUS ARTICLE or to Kaiser Aluminum & Chemical Corporation, a corporation of Delaware No Drawing. Application January 2, 1952, Serial No. 264,662

8 Claims. (Cl. 29-1962) This invention relates to composite metal articles having a ferrous metal base portion coated with a particular aluminum base alloy, the process for producing such composite articles and the aluminum base coating alloys. More particularly. the invention relates to aluminum alloys particularly adapted for the hot dip coating of a ferrous metal base. the hot dip process and the resulting composite metal article.

Coatings of aluminum on iron and steel products are highly desirable, since the composites resulting in effect embody the superior properties resident in each metal. To the strength and other desirable characteristics of the iron or steel core, the aluminum coating adds the more salient properties of resistance to corrosion and oxidation at both atmospheric and elevated temperatures, enhanced electrical conductivity, improvement in the facility of brazing and soldering of aluminum to steel, and a more attractive appearance.

In the fabrication of aluminum coated ferrous base metal products, it is often necessary that the composite article exhibit the ability to withstand high degrees of deformation upon subsequent working or forming. The coating upon the ferrous base in such cases must possess satisfactory adherence so that upon bending or drawing or other deformation the coating will not break away, that is peel or flake off the base metal. in addition, the composite of the ferrous base and aluminum coating, as well as the coating itself mus-t exhibit satisfactory ductility so that cracking of the coating and exposure of the base metal to corrosion will not occur. In addition, such coatings to be commercially acceptable must exhibit a satisfactory appearance in regard to coverage on the ferrous base, for example wire or strip, continuity and smoothness of the coating and the luster or attractiveness of the coating.

Of the several processes for the production of aluminum coated ferrous base composite articles, the hot dip method is the most desirable from an economic standpoint and from the standpoint of range of applicability. However, the production of aluminized (aluminum-coated) ferrous base metal articles has presented many problems even in the advantageous hot dip method to which the present invention is particularly directed.

In hot dip coating ferrous metal articles with high purity or commercially pure aluminum, the principal dithculties encountered have been the lack of satisfactory adherence of the coating to be basis metal, lack of ductility of the composite and coating, and to tendency to produce large grains which are visible to the naked eye. These deficiencies have seriously interfered with the utility of such hot dip aluminum coated articles, since the formability of the composite in regard to subsequent working is unsatisfactory due to cracking, peeling or flaking of the coating upon deformation of the composite. In addition, cracking of the coating leads to unsatisfactory in service performance in regard to the intended purpose of the coating to enhance the appearance and corrosion resistance of the ferrous base. Accordinglgfor the most part 2,782,493 Patented Feb. 26, 1957 only the color characteristic of the high purity or commercially pure aluminum coating on the ferrous base has proved to be satisfactory.

Attempts have been made to counteract the rather poor adherence and ductility characteristics of aluminum coated ferrous metal composite articles wherein high purity or commercially pure aluminum has been employed as the coating metal or alloy. It has been determined that the ductility of the coating and composite is primarily adversely affected by the thickness and hardness of the inter-metallic brittle Fe-Al phase layer formed between the core and the superimposed or outer layer of the aluminum-rich coating metal upon immersion of the basis metal into the molten bath of coating metal. With commercially pure or high purity aluminum as the coating metal, the inter-metallic or interfacial Fe-Al layer grows relatively rapidly by diffusion at normal hot dip coating temperatures so that the composites exhibit an undesirably low ductility and adherence.

It has heretofore been recognized that the addition of silicon in various amounts to a hot dip aluminum coating bath advantageously effects a control on the formation and growth of the inter-metallic Fe-Al alloy layer so that although the layer forms in the proper thickness to provide the requisite adherence or bonding between the coating and the ferrous base, its thickness is greatly rcdueed in comparison to that when utilizing high purity or commercially pure aluminum. However, the extent to which the silicon effectively reduces the thickness of the inter-metallic or alloy layer to produce truly significant increases in ductility of the composite usually requires the addition of three percent or more silicon based on the weight of the aluminum-silicon alloy utilized as the coating alloy. Amounts of silicon as low as about one percent will produce a decrease in inter-metallic alloy layer thickness, but the increase in ductility and adherence is much less marked than where the silicon is employed in the larger amounts. Thus, satisfactory adherence and ductility may be obtained when utilizing aluminum-silicon alloys in the hot dip coating process. Unfortunately. however, the addition of silicon, particularly in the amounts required to produce significantly high properties of ductility causes an undesirable appearance of the coating in regard to both luster and smoothness. Also the continuity of the coating tends to decrease with the larger amounts of silicon since the molten coating bath becomes increasingly sensitive to minor variations in the cleanliness of the ferrous base. The silicon-containing aluminum alloys provide a characteristically grey. non-lustrous appearance, which becomes markedly roughened in surface characteristics where the silicon is used in the most favorable amounts in regard to the ductility and adherence characteristics of the composite articles.

It is accordingly a primary purpose and object of this invention to provide hot dip aluminum alloy coated ferrous base metal articles which combine the several properties of good adherence. ductility and excellent appear ance in regard to coverage, smoothness, continuity and luster of the coating. and other factors affecting appearance.

it is a further object of the invention to provide aluminum base alloys particularly adapted for the hot dip coating of ferrous base metals characterized by their ability to form coatings and composite articles exhibiting good properties of form-ability due to high ductility and adherence and desirable appearance of the coatings in regard to brightness, smoothness and continuity and ab sence of undesirable visible surface grains.

It is a more specific object of the present invention to provide aluminum alloys for coating ferrous metal wherein the advantages of good adherence and ductility charac-- terized by silicon-containing coating alloys are obtained while at the same time the disadvantages in regard to oating appearance characterizing aluminum-silicon binary coating alloys are substantially eliminated.

' of the invention to provide a It hot dip aluminum coatings on ferrous base metal and may be obtained by the utilization of certain ternary and more complex aluminum base alloys. In general the aluminum base alloys of the invention about I to 6% silicon, and small but effective amounts of at least one of the elements boron, titanium, vanadium and zirconium; particularly the first two named and last named of these elements, employed preferably singly, or in some instances collectively; to produce the abovemcntioned desired results. It was found that the presence of boron, titanium, vanadium and zirconium decidedly loys, particularly wherein the silicon was present in the higher range of amount. The coatings obtained were smoother and characterized by the absence of the relatively rough steel grey coating typical of those produced by the aluminumsilicon binaries. In addition, the appearance of the coatings was enhanced by the production These additional elements, in particular boron, titani um, and zirconium also exerted a effect upon the ductility of the alloy coating on the ferrous base core. In this regard, photomicrographical studies indicated that aluminum-silicon binaries, particularly in the higher range of silicon, that is, 4 to 5 and over 5% silicon, produced a structure in which the intermetallic compounds, that is, the Fe-Al phase layer penetrated the alloy coating in finger-like form. This was the reverse of the usual structure. For example, when utilizing high purity aluminum as metallic layer penetrated the ferrous base core and the ally relatively that the alloying elements of the present invention tended to increase the ductility of the alloy coating which apsdversely affected by the finger-like protrusions of the intermetallic layer into the coating in the case of the aluminum-silicon binaries of relatively high silicon content.

increasing the ductility of the coating and greatly im proving the appearance of the coating. With aluminumsilicon binaries of lower silicon content, the boron, titanium, vanadium and/or zirconium exerted a beneficial significant beneficial elfect on the ductility of the composite while contributing to an improvement in the appearance of the surface of the coating by reduction of grain size.

As indicated above, although the invention embraces the use of these elements collectively, it has been found preferable for optimum results to employ the elements However, boron, which produces the advantages of the other elements above noted, in addition, may advantageously be used to boron and one of the elements titanium, vanadium and zirconium.

Additions of boron and titanium are recommended in amount of from about 0.02 to about 0.20% by weight the preferred amounts of from about 0.02 to 0.05% boron, and from about 0.04 to 0.18% titanium. The other two elements, namely vanadium and zirconium are recommended in amounts from about 0.1 to about 0.25% on an individual basis. Where two or more of the elements are used collectively, and as a rule not more than two such elements are generally used, the total should not exceed about 0.5%.

As a further feature of the present invention, it was determined that aluminum-silicon coating alloys containing the addition elements above mentioned, were definitely benefited by the addition of the element zinc. In addition, it was noted that these alloys containing zinc exhibited an effect wherein the influence of the elements of the boron, titanium, vanadium and zirconium group complemented that of the influence of zinc. It was found that the presence of the one counteracted some of the disadvantages of the other.

The addition of zinc in The presence of zinc overcame the occasional tendency of the coating alloys of the invention to produce coatings having roughened surfaces due to high melting compounds on the ferrous base article. In addition, the presence of zinc exhibited the decidedly beneficial effect of at least partially offsetting the characteristic steel grey appearance imparted by the silicon constituent, particularly when present in the higher range of amounts. Thus, the luster, brightness and color of the coatings were enhanced by the presence of zinc. Also, the zinc contributed along with the titanium, boron, vanadium and zirconium in increasing the smoothness of the surface of the alloy coating.

In general, additions of zinc in amount from about 0.5 to 1% are to be preferred. However, it was noted that amounts of zinc up to about 2% could advantageously be used, while the tendency toward a spangling effect in the appearance in the coating was substantially olfset by the presence of boron, titanium, vanadium and/0r zirconium. In exerting the beneficial effects above noted, zinc does not impair the ductility characterizing the alloys of the present invention. It appears that the zinc constituent further provides cathodic protection for any portion of the basis metal of iron or steel which might become exposed in service should the coating develop any cracks.

The alloys of the present invention as above described, produce composite articles of aluminum alloy coated ferrous metal having a ductility of the composite equal to that of an aluminum-silicon binary containing silicon in the higher range of amounts, and alloy coatings the appearance of which is at least equal to and in most instances better than that obtained with high purity aluminum. In addition, these coatings exhibit ductilities better than that of high purity aluminum.

It is to be emphasized in regard to the coating alloys of the present invention that the selection of a given alloy for aluminizing by hot dip methods should be governed by the application for which the composite is intended. As an example, several of the specific alloys included in the present invention which could be said to have general purpose utility are in the following table.

TABLE 1 Alloys best suited for general purpose bining properties good ductility, and operational characteristics of the molten applications (comgood appearance,

bath).

Nominal composition in percentage of alloying constituents based on weight of total alloy:

1. silicon-0.20% zirconium. 2. 5% silicon-1% zinc-0.12% titanium.

3. 5 silicon2% zinc-0.18% titanium. 4. 5% silicon-0.5% zinc-0.12% titanium.

Ductility Point Ratings I Nominal composition in percentage of alloying constituents based on weight of alloy (in ascoated condition) 180 Bend Spiral Wrap est Test 5% silicon-0.207 zirconium 1 2 5% silic0nl% z nc-0.12% titanium. 1. 52 2. 5 5% silicon-2% zinc-0.18% titanium. 1 2. 5 5% silicon-0.5% zinc-0.12% titanium 1. 5-2 2 Reference Alloys: l i

99.9% aluminum 4 i 4 2S (commercially pure aluminum). 4. 5 i 5 5% silicon binary aluminum alloy 1. 5 2

1 In each oi the above tests, the lei-mus stool: specimens were mild stcv strip 12 inches in length, 0.25 inch wide and 0.032 inch thick. In the 180" bend test, the radius of bond was 0.10 inch, and in the spiral wrap test the radius of bond was 0.070 inch. The point ratings are relative to the test behavior 0! 99.9% aluminum in the as-coated condition which is arbitrarily assigned the value cl 4 for each bend test. The rating is such that the lower the point rating, the more desirable is the coating alloy and thrmeanlngs attached to the various point ratings are as follows:

1=Nil to very slight; wrinkling on the inner or compression side of the coated stri with no cracking. 2=Slight wrinkling on the inner or compression side of the coated strip with no cracking.

of the coated 3=Moderate wrinkling on the inner or compression side strip with no cracking.

4=Marlred wrinkling on the inner or compression side of the coated strip with slight cracking and spelling on the outer or tension side of the coated strip.

It is readily seen from the above rating chart that the aluminum alloys of the invention in the as-coated condition are superior in ductility to the more pure aluminum coating alloys such as 28 alloy or 99.9% aluminum alloy. It is also to be noted that the aluminum alloys of the invention in the as-coated condition were found to have a ductility rating substantially equivalent to that of the due tile 5% silicon aluminum binary alloy in the as-coaied condition.

With respect to the improved appearance of the coating alloys of the invention as compared to the reference alloys, it was found that the surfaces of the specimen coated with 99.9% aluminum were mottled in appearance due to the visible large grain structure of the coating and the coverage of the coating at the edges of the strip tended to be incomplete, or where continuous, it was tough The coating of commercially pure aluminum (25 alloy) was also characterized by a visible large grain, which, however, was somewhat obscured by a more or less opaque oxide film which presented a relatively dull appearance compared to a metallic luster. The coating on the edges of the strip was rough and continuity was slightly lacking.

The 5% silicon-aluminum alloy coating had a dull greyish color seriously lacking in luster, and was characterized by a markedly rough surface texture over substantially the whole strip, although the coating was continuous on the strip edges.

The alloy examples of the invention as set forth above in Table I, however, when used as coating materials for ferrous base metals produced composites of improved appearance and continuity as evidenced by the excellent coverage, smooth grainless texture and luster of the coatings compared to conventional high purity and commercially pure aluminum coatings. and vastly improved appearance as contrasted with the silicon-aluminum binary alloys.

The aluminum alloys of the invention embrace those coating materials which may be designated as the ternary and more complex alloys set forth above wherein the bal ance of the alloy is substantially all aluminum and impurities in normal amounts. The term impurities in normal amounts as used herein and in the appended claims includes the iron which may be progressively picked up by the molten aluminum alloy bath from the ferrous "base during continued operation, as discussed hereinbelow. Other elements in the form of residuals or impurities such as copper may be present in normal impurity amount without unduly adversely affecting the beneficial results obtained by the combination of silicon and the other alloying constituents of the invention with the aluminum coating metal.

In utilizing the alloys of the present invention in the hot dip process various procedural modifications may be employed. Thus, the process may be conducted on either a batch or continuous basis. depending primarily on the nature of the article to be coated. For example, with wire, strip or sheet, the continuous method is usually employed, whereas with individual work pieces such as castings and the like, the batch process may be preferable. In either case, the coating alloys and molten baths prepared therefrom produce aluminum alloy coated ferrous metal articles superior to similar articles coated with high purity aluminum (99.9% commercially pure aluminum (2S) and even aluminum containing 5% or more silicon.

The ferrous articles to be coated are first thoroughly cleaned by any suitable method, such as acid pickling to remove oxide film or scale. They may then be rinsed and dried, and immersed in the coating bath with or Without the use of a conventional flux. For large sections. the use of flux may be desired for preheating and further cleaning before immersion in the alloy bath. On the other hand, the articles with or without pickling depending upon the nature of the surface may be pretreated by bright annealing in suitable apparatus in an inert or reducing atmosphere and then directly immersed in the molten coating bath without exposure to the atmosphere.

The bath temperature is maintained suliiciently high so that the alloys employed are completely molten. Of course, the particular bath temperature depends upon the coating alloy composition and the composition and nature of the ferrous article; and in regard to the alloys herein disclosed temperatures of from about 1280 to about l375 F. are recommended.

Since thickness of the Fe-Al interfacial layer increases with bath temperature, it is recommended that the lowest operating temperature consistent with good coating results be used. It is an advantage of the invention however that the rate of increase in thickness of the Fe-Al layer with temperature is lower with the alloys herein disclosed than with pure aluminum.

The time of immersion depends principally upon the composition of the molten bath, the composition and nature of the ferrous article, and the temperature of the molten bath. The immersion time and also the bath temperature may be regulated to produce a composite suitable for the intended application. Extending the time of immersion tends to increase the thickness of the Pe-Al layer and accordingly the shortest immersion times consistent with satisfactory coverage are usually recommended.

Example As an example of one specific procedure in utilizing the coating materials of the invention to produce composites by the hot dip method, the following is set forth as illustrative without constituting a limitation on the scope of the disclosure and claims appended hereto:

A specimen of mild steel strip containing 0.2% manganese and less than 0.1% carbon, 12 inches in length. 0.25 inch wide and 0.032 inch thick was used. The specimen was degreased with carbon tetrachloride and wiped dry with cloth, then pickled in a 40% HCl solution for from 15 to 25 seconds at a temperature of l40l60 F. This was followed by a water rinse and the cleaned specimen was dried with gauze.

The specimen was then dipped in a molten bath of aluminum base alloy containing silicon, 2% zinc, and 0.18% titanium, the balance aluminum and impurities in normal amounts. No fluxes were employed, and the specimen was immersed for from to 30 seconds with the bath temperature maintained at about 1300 F. The coated specimen was then withdrawn and tapped tightly while in a vertical position to facilitate removal of the excess coating.

It is to be noted that during the course of operation etiher batch or continuous, the molten aluminum alloy bath gradually increases in iron content due to pickup by dissolution of iron from the basis metal. Small amounts of iron in the coating alloy up to about 2 /z% may be tolerated without any significant adverse eifect on the character of the coating and composite. However, when the iron exceeds this amount, for example, approaches saturation on the order of about 3% at the usual coating temperatures of the bath above referred to, various procedures may be adopted for reducing the iron content. Among such procedures may be mentioned discontinuance of operation of the bath while permitting the heavy iron component to settle out of the molten aluminum alloy. or addition of fresh amounts of pure aluminum alloy to effectively reduce the concentration ot'iron in the total molten bath.

As a result of the present invention, coating alloys have been developed for aluminum hot dip coating of ferrous metal articles whereby composites of ductilities and adherence equal to that of aluminum-silicon binaries are produced. while at the same time the aluminum alloy coatings on the composites exhibit appearances which are at least equal to and in many instances better than that of high purity aluminum coatings.

All percentages in the claims are by weight of the total alloy.

What is claimed is:

l. A composite article comprising a base portion of ferrous metal coated with an aluminum base alloy consisting essentially of from about 1 to about 6% silicon and at least one element selected from the group con- 8 sisting of boron, titanium, vanadium and zirconium in amount from about 0.02 to about 0.20% boron and titanium, and about 0.1 to 0.25% vanadium and zirconium, the total not exceeding about 0.5%, the balance substantially all aluminum and impurities in normal amounts.

2. A composite article according to claim 1 in which the aluminum base alloy also contains from 0.5 to 2% zinc.

3. A hot dip coated product comprising a ferrous metal base coated with an aluminum base alloy consisting essentially of from about 1 to 6 0 silicon, and one element selected from the group consisting of boron, titanium, vanadium, and zirconium in amount from about 0.02 to about 0.20% boron and titanium, and from about 0.1 to about 0.25% vanadium and zirconium, balance aluminum and impurities in normal amounts.

4. A hot dip coated product according to claim 3 in which the aluminum base alloy also contains from about 0.5 to about 2% zinc.

5. A composite article comprising a base portion of ferrous metal coated with an aluminum base alloy consisting essentially of from about 1 to about 6% silicon and at least one element selected from the group consisting of boron, titanium, vanadium and zirconium in amount from about 0.02 to about 0.05% boron, from about 0.04 to about 0.18% titanium, and from about 0.1 to about 0.25% vanadium and zirconium, the total of these elements not exceeding about 0.5%, the balance substantially all aluminum and impurities in normal amounts.

6. A composite article according to claim 5 also containing zinc in amount from about 0.5 to about 1%.

7. A composite article comprising a base portion of ferrous metal coated with an aluminum base alloy consisting essentially of from about 1 to about 6% silicon and one element selected from the group consisting of boron, titanium, vanadium and zirconium in amount from about 0.02 to about 0.05% boron, from about 0.04 to about 0.18% titanium, and from about 0.1 to about 0.25% vanadium and zirconium, balance aluminum and impurities in normal amounts.

8. A composite article according to claim 7 also con taining zinc in amount from about 0.5 to about 1%.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A COMPOSITE ARTICLE COMPRISING A BASE PORTION OF FERROUS METAL COATED WITH AN ALUMINUM BASE ALLOY CONSISTING ESSENTIALLY OF FROM ABOUT 1 TO ABOUT 6% SILICON AND AT LEAST ONE ELEMENT SELECTED FROM THE GROUP CONSISTING OF BORON, TITANIUM, VANADIUM AND ZIRCONIUM IN AMOUNT FROM ABOUT 0.02 TO ABOUT 0.20% BORON AND TITANIUM, AND ABOUT 0.1 TO 0.25% VANADIUM AND ZIRCONIUM, THE TOTAL NOT EXCEEDING ABOUT 0.5%, THE BALANCE SUBSTANTIALLY ALL ALUMINUM AND IMPURITIES IN NORMAL AMOUNTS. 